Week 3 Content – Imperative Programming in Java, Part 2

Week 3 Content – Imperative Programming in Java, Part 2 Video 1 – Methods This is a statement in which we call a method called System.out.println with a single string parameter "hello". Here we are declaring & initialising a variable (of type double) called twoSquared. To do this, we set the value as the result of calling the function Math.pow with two double parameters of 2.0 and 2.0. What is a Method? A method is a code block with a name. They: • Are defined within a class definition • Can have arguments and a return value • Correspond to functions or procedures in other languages Defining a Method For example: • • • • • The method is defined within the class MaximumDemo public static – modifiers which change the behaviour of the method int – type of the return value maximum – name of the method being defined (int a, int b) – types and names of arguments The main method: Java programs require that each class contain a method identified as main which is run by default by the compiler, and is usually used to call another method within that same class. Note that the main method is declared in the same way as we would declare any other method, with its only use being that it will be called by the interpreter as default. We can access any parameters given via the command line via args: Using the command java Hello Liam Ward to call this Hello class from the terminal would return "Hello Liam!", since we are only using the first parameter as given in the command line. Method Signatures A method is uniquely identified by its signature: the modifiers, spelling of the identifier, types and ordering of the parameters form the signature of the method. All of these method declarations are different: Video 2 – Loops For loops A "for loop" is used to repeat a block of code a fixed number of times: • Initialization declares a variable (usually of a numerical data type) which is used to 'keep track' of how many times the code within the loop has been repeated • The condition 'decides' whether another iteration of the loop should be performed or not • Update – as the name suggests – updates the variable declared in the initialization with every iteration of the loop Examples: This is a very simple For loop which prints the first 10 integers to the screen, each on a different line. We can see that we have a variable – i – which is initialised to 1, the condition is to continue until i = 10, and we increment I by 1 with every iteration. This loop prints every fifth element between 0 and 1000 (inclusive). We initialise a variable k which = 0, set our condition to stop when k = 100 and increment k by 5 with every iteration. This code first initialises an integer variable current, which is set to 3. There is then a for loop which initialises a variable k as 1, the condition k <= 10 and increments k by 1 every time. This code is simply a more elegant implementation of the loop above. While loops A while loop can be seen as a sub-type of a for loop, where we forgo the initialisation & update statements, leaving only a condition. The code within the body of the loop repeats so long as the condition evaluates to True. These kinds of loops are useful where we do not know how many times a loop will have to repeat. Example: Video 3 – Imports, Packages & the Classpath When we are writing a very large project with lots of classes, we can have the problem of having a cluttered namespace: that is to say, we run out of new names for classes. Luckily, Java allows us to sort classes into packages of related classes. Packages A package is a collection of related classes • The full name of a class in a package is package.class • Packages can be nested • Packages from the standard library start with java. For example, the System class is in a package called java.lang, so the full name of the System class is java.lang.System Import The import statement introduces a class to the current namespace: Now that we have imported the full class, we can refer to it only by its class name. Note that this is for convenience: we can still refer to java.util.Scanner without importing. Apropos Convenience The java.lang package is automatically available to all classes without using the import statement. This is why we can refer directly to system when using System.out.println, rather than having to use its full package name: java.lang.System.out.println Creating Packages To assign a class to a package, we need to do two things: 1. Add a package statement to the source code 2. Move the source file for the class we wish to include into a directory mypackage/. Both the compiler and interpreter will look for class files in a directory named just like their package. Where does the compiler & interpreter start looking for packages? In the classpath! The Classpath The classpath is a list of directories from which packages are looked up. • Both javac and java both have command-line arguments to set the classpath • The current directory is automatically added to the classpath • We can use environemnt variables to set the classpath (simply CLASSPATH on Unix-based systems) Conventions • Package names are usually all lower case to avoid conflict with class names • It is common to use reversed domain names for packages. This give us globally-unique package names which avoid naming collisions between libraries from different sources.